It is well known to add additives, e.g. calcium compounds, to carbonaceous material being gasified in a slagging gasifier thereby to decrease the ash fusion temperature. However, in the case of fixed bed dry bottom gasifiers such as the Sasol-Lurgi fixed bed dry bottom gasifier, the slagging of ash is undesired as it leads to unstable operation or inoperability of the gasifier. A fixed bed dry bottom gasifier must thus be operated in a temperature region such that the maximum gasifier temperature is below the ash fusion temperature of the carbonaceous material which is being gasified. Conventionally, this is achieved by decreasing the oxygen load into the gasifier or by operating the gasifier with an excess of steam as gasification or moderating agent. Decreasing the oxygen load into the gasifier is undesirable as it results in a direct reduction in synthesis gas production. Operating the gasifier with an excess of steam is also not ideal as it results in decreased thermal efficiency of the gasification process as more energy is required to generate the excess steam.
Fixed bed dry bottom gasifiers such as the Sasol-Lurgi fixed bed dry bottom gasifiers are also known as moving bed dry ash gasifiers.